[PATCH rfc] nvme-pci: make sure to flush sqe writes before db record update

[Sagi Grimberg]

A wmb() is always required before operations that trigger DMA
(e.g. mmio write to a doorbell record). Not having it might result
in write reordering causing the device to fetch partly updated
submission queue entries.

Reported-by: Jason Gunthorpe <jgg at ziepe.ca>
Signed-off-by: Sagi Grimberg <sagi at grimberg.me>
---
Note that this is stable material, We could add stable
tag when applying it to the nvme tree.

 drivers/nvme/host/pci.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index db4dedb1cc8f..6c2dcef1ff91 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -437,8 +437,14 @@ static void __nvme_submit_cmd(struct nvme_queue *nvmeq,
 	if (++tail == nvmeq->q_depth)
 		tail = 0;
 	if (nvme_dbbuf_update_and_check_event(tail, nvmeq->dbbuf_sq_db,
-					      nvmeq->dbbuf_sq_ei))
+					      nvmeq->dbbuf_sq_ei)) {
+		/*
+		 * Make sure that descriptors are written before
+		 * doorbell record.
+		 */
+		wmb();
 		writel(tail, nvmeq->q_db);
+	}
 	nvmeq->sq_tail = tail;
 }
 
-- 
2.14.1